Silver-to-silver bonded IC package having two ceramic substrates exposed on the outside of the package

ABSTRACT

A packaged power device involves no soft solder and no wire bonds. The direct-bonded metal layers of two direct metal bonded ceramic substrate assemblies, such as Direct Bonded Aluminum (DBA) substrates, are provided with sintered silver pads. Silver nanoparticle paste is applied to pads on the frontside of a die and the paste is sintered to form silver pads. Silver formed by an evaporative process covers the backside of the die. The die is pressed between the two DBAs such that direct silver-to-silver bonds are formed between sintered silver pads on the frontside of the die and corresponding sintered silver pads of one of the DBAs, and such that a direct silver-to-silver bond is formed between the backside silver of the die and a sintered silver pad of the other DBA. After leadforming, leadtrimming and encapsulation, the finished device has exposed ceramic of both DBAs on outside package surfaces.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority under 35U.S.C. §120 from, nonprovisional U.S. patent application Ser. No.14/144,587 entitled “Silver-to-Silver Bonded IC Package Having TwoCeramic Substrates Exposed on the Outside of the Package,” now U.S. Pat.No. 8,987,911, filed on Dec. 31, 2013, the subject matter of which isincorporated herein by reference. Application Ser. No. 14/144,587, inturn, claims priority under 35 U.S.C. §119 from U.S. ProvisionalApplication No. 61/748,025, entitled “Silver-To-Silver Bonded IC PackageHaving Two Ceramic Substrates Exposed On The Outside Of The Package,”filed on Dec. 31, 2012, the subject matter of which is incorporatedherein by reference.

TECHNICAL FIELD

The described embodiments relate to integrated circuit packaging forpower devices.

BACKGROUND INFORMATION

A type of integrated circuit package is known that involves a powerintegrated circuit die or dice sandwiched between the inner surfaces oftwo opposing parallel-oriented Direct Bonded Copper (DBC) substrates.U.S. Pat. Nos. 6,812,533 and 7,697,303 and U.S. Patent Publication2012/0314372 set forth examples of packages in which a power integratedcircuit die is sandwiched between two such ceramic substrate members.Stamped metal leads of the package extend from between the two substratemembers. An improved integrated circuit package is sought.

SUMMARY

A packaged power device involves no soft solder, nor does it involve anywire bonds. A direct metal bonded ceramic substrate, such as a DirectBonded Aluminum (DBA) substrate, has islands of metal that aredirect-bonded to a ceramic substrate. The islands of two such DBAs areprovided with sintered silver pads. The two DBAs are attached to aleadframe by ultrasonic welding.

Silver nanoparticle paste is applied to aluminum bond pads on thefrontside (i.e., device side) of a die and the paste is sintered so thatthe aluminum bond pads on the front side of the die are covered withsintered silver pads. Special nanoparticle paste may be used thatpenetrates a thin native oxide present on the aluminum bond pads duringsintering such that good mechanical and electrical contact is madebetween the sintered silver structure and the underlying aluminum pad.Silver formed by an evaporative process covers the entire backside ofthe die.

Multiple such dice are placed onto the DBAs such that silver on thebackside of each die is in contact with a corresponding sintered silverpad of a DBA. The resulting assembly is then folded so that theleadframe bends, and so that the dice are pressed between the two DBAs.For each die of the assembly, the pressing causes a directsilver-to-silver bond to be formed between the evaporative silver on thebackside of the die and a corresponding sintered silver pad of one ofthe DBAs, and such that direct silver-to-silver bonds are formed betweensintered silver pads on the frontside of the die and correspondingsintered silver pads of the other DBA. Alternatively, the dice can beattached so that their silvered backsides are attached to theirrespective DBA in a first step, and then the assembly is folded in asecond step, and then pressure is applied to attach the sintered silverpads on the frontsides of the dice to their corresponding BDAs in athird step.

The resulting folded structure is then encapsulated with an amount ofinjection molded encapsulant so that a body portion of the packagedpower device is formed. Leadforming and leadtrimming are carried out toform a single row of in-line package terminals that extends from oneside of the body portion of the package. The finished packaged powerdevice has exposed ceramic of both DBAs on outside surfaces of the bodyportion of the package. In one example, the packaged power device has nosoft solder, nor does it have any bond wires.

Further details and embodiments and techniques are described in thedetailed description below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 is a flowchart of a method in accordance with one novel aspect.

FIG. 2 is a top-down perspective view of a wafer of power devices.

FIG. 3 is a partial cross-sectional diagram of one die area of the waferof FIG. 2.

FIG. 4 is a partial cross-sectional diagram that illustrates anapplication of silver nanoparticle paste onto aluminum pads on thefrontside of the die area of FIG. 3.

FIG. 5 is a top-down diagram of the wafer after application the silvernanoparticle paste.

FIG. 6 is a table that sets forth the constituents of the silvernanoparticle paste applied to the wafer, in one specific exemplaryembodiment.

FIG. 7 is a perspective diagram of two of the dice that are formed bydicing the wafer of FIG. 5.

FIG. 8 is a perspective diagram of two Direct Bonded Aluminum (DBA)substrate assemblies that are interconnected by metal of a leadframe,where sintered silver pads are provided on the aluminum islands of theDBAs.

FIG. 9 is a perspective diagram that illustrates the attaching of thetwo dice of FIG. 7 to the two DBAs of FIG. 8 using solderlesssilver-to-silver bonding.

FIG. 10 is a perspective diagram that shows the structure of FIG. 9being folded so that the sintered silver pads on the frontsides of thedice can be direct silver-to-silver bonded to corresponding sinteredsilver pads on the islands of the DBAs.

FIG. 11 is a cross-sectional diagram that illustrates how the foldedstructure of FIG. 10 is pressed together under adequate pressure andtemperature, and for an adequate amount of time, such that directsilver-to-silver bonds are formed between the sintered silver pads onthe frontsides of the dice and corresponding sintered silver pads of theDBAs.

FIG. 12 is a cross-sectional diagram that shows the packaged powerdevice after leadforming, leadtrimming and encapsulation.

FIG. 13 is a circuit diagram of the packaged power device of FIG. 12.

FIG. 14 is a perspective diagram of the packaged power device of FIG.12.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. In the description and claims below, when a first object isreferred to as being disposed “over” or “on” a second object, it is tobe understood that the first object can be directly on the secondobject, or an intervening object may be present between the first andsecond objects. Similarly, terms such as “front”, “back”, “top” and“bottom” and similar terms are used herein to describe relativeorientations between different parts of the structure being described,and it is to be understood that the overall structure being describedcan actually be oriented in any way in three-dimensional space.

FIG. 1 is a flowchart of a method 100 in accordance with one novelaspect. Silver is deposited (step 101) on the backside of a wafer usingan evaporative deposition process. Silver covers the entire backside ofthe wafer or substantially all of the backside of the wafer. FIG. 2 is atop-down diagram of the wafer 1. FIG. 3 is a cross-sectional side viewof one die area of the wafer of FIG. 2. In the present example, each diearea includes a transistor, such as an Insulated Gate Bipolar Transistor(IGBT). Aluminum pads 2 and 3 are disposed on the frontside of die area.Aluminum contact pad 2 is for connection to the gate of the transistor.Aluminum contact pad 3 is for connection to the emitter of thetransistor. The silver 4 on the backside of the wafer is for contact tothe collector of the transistor.

Sintered silver pads are formed on the frontside of the wafer so thatthere is a plurality of sintered silver pads in each die area of thewafer. FIG. 4 is a diagram that shows microdots 5 of silver nanoparticlepaste 7 being ink-jet printed down onto the aluminum contact pads 2 and3 on the frontside of the die area. The silver nanoparticle paste 7 isonly applied to the aluminum contact pads and is not applied elsewhereon the frontside of the wafer. In the illustrated example, the printingis applied in a micro-jetting process that involves use of an ink-jetmicronozzle head 6. The ink-jet micronozzle head is scanned back andforth across the wafer and the micronozzle is controlled to emitmicrodots of the silver nanoparticle paste onto only the aluminumcontact pads. In another example, the printing is not an ink-jetprinting process but rather is a screen printing process involving astencil.

FIG. 5 is a top-down diagram of the wafer 1 after application of thesilver nanoparticle paste 7 to the aluminum pads on the frontside of thewafer. Each of the squares is the boundary of one of the die areas. Thealuminum pads are illustrated as shaded to indicate that each aluminumpad is covered with silver nanoparticle paste.

FIG. 6 is a table that sets for the constituents of the silvernanoparticle paste 7 that is applied onto the aluminum pads, in onespecific embodiment. For further details including details on the silvernanoparticle paste of FIG. 6, on other suitable sintering pastesinvolving silver particles, on related techniques, and on how to usesuch silver particle pastes in carrying out solderless die attach to aDBA, see: U.S. Pat. No. 8,716,864, entitled “Solderless Die Attach To ADirect Bonded Aluminum Substrate”, filed Jun. 7, 2012 (the entirecontent of which is expressly incorporated herein by reference).

After application of the silver nanoparticle paste, the wafer 1 isheated so that the individual volumes of silver nanoparticle paste onthe frontside of the wafer are sintered to form individual sinteredsilver structures. As the temperature increases to about 150° C., thethinner in the paste evaporates. This results in a somewhat more densepacking of the silver nanoparticles. Once the thinner has evaporated,the temperature is increased further to approximately 200° C. The typeof dispersant/binder is selected so that the dispersant/binder coatingseparates from the silver particles and burns out at this 200° C.elevated temperature, but before actual sintering takes place at ahigher sintering temperature of about 250° C. Generally the dispersantand the binder are organic molecules involving carbon chains of twelveor more carbon atoms, whereas the thinner is an organic molecule thathas carbon chains of approximately three carbon atoms. Before burningoff, these constituents and the flux particles in the paste decompose toform reactive compounds and acids. The flux particles are not active atroom temperature, but under the elevated temperatures before burn offthe flux particles decompose into compounds such as acids. Thesecompounds attack and help penetrate any thin native aluminum oxide thatis present on the aluminum pads. Pressure and ultrasonic energy can alsobe provided to promote cracking of the thin native aluminum oxide. Careis taken, however, not to be so aggressive that damage comes to thestructure underlying the aluminum pads. Bonding between the silvernanoparticles and the exposed aluminum, and/or between the otherconstituents of the paste and the exposed aluminum, occurs in thesecracks in the thin native oxide. As the temperature continues toincrease, the dispersant/binder and flux residue are burned off ordriven off as a consequence of the higher sintering temperatures at theend of the sintering process. In an example where no pressure isapplied, sintering occurs at a temperature of 250° C. Alternatively,pressure-assisted sintering is used in combination with smaller silverparticles so that sintering occurs at a lower temperature that is higherthan 200° C. but lower than 250° C. After sintering, the frontside ofthe wafer is thoroughly cleaned to remove any flux residue that mightstill remain.

The cleaned wafer is then diced (step 103) thereby forming a pluralityof semiconductor dice. FIG. 7 is a perspective diagram of two of thedice 8 and 9. First die 9 has two sintered silver contact pads 13 and 14on its frontside and also has a layer 15 of silver on its backside.Second die 8 has two sintered silver contact pads 10 and 11 on itsfrontside and also has a layer 12 of silver on its backside.

A first direct metal bonded ceramic substrate assembly 16 has aplurality of islands 17-19 of aluminum. The islands of aluminum aredirect bonded to a thicker ceramic substrate portion 20. Similarly, asecond direct metal bonded ceramic substrate assembly 21 has a pluralityof islands 22-24 of aluminum that are direct bonded to the ceramicsubstrate portion 25. The aluminum islands are made by pressing analuminum plate onto a larger ceramic panel. The larger ceramic panelmay, for example, be 5.6 inches wide by 7.7 inches long, and may be 0.63mm thick. The aluminum plate may be 0.30 mm thick. After attachment ofthe aluminum plate to the ceramic substrate panel, the aluminum plate ispatterned and etched to form the individual aluminum islands. Allexposed aluminum surfaces are then cleaned (for example, using a strongalkaline solution of NaOG followed by a zinc-based treatment) and areimmediately plated with a metal such as nickel or palladium. Anelectroplating process may be used. The resulting plating metal layerson the aluminum islands are about five microns thick. Alternatively, anelectroless nickel plating process may be used in which case the nickellayer may include about eight percent phosphorous. Alternatively, anelectroless palladium plating process may be used. Multiple layers ofmetal can be plated. The topmost metal plating layer is a metal thatwhen silver nanoparticle paste is sintered upon it, forms a good bondwith the sintered silver. For further details on the manufacture ofdirect metal bonded substrate assemblies, see: U.S. Pat. No. 6,798,060,U.S. Pat. No. 7,005,734, and U.S. Pat. No. 6,404,065 (the subject matterof which is incorporated herein by reference).

Silver nanoparticle paste is then applied onto selected areas of thesemetal-plated aluminum islands. Where the metal onto which the silvernanoparticle paste is applied is palladium, the silver nanoparticlepaste may be mAgic Paste Microbond paste, series ASP016, ASP043, ASP131or APA859, that is commercially available from Heraeus MaterialsTechnology GmbH & Co. KG of Hanau, Germany. A screen printing processusing a patterned screen and a wiper may be used. The silvernanoparticle paste on the aluminum islands of the DBAs 16 and 21 is thensintered to form the silver pads 26-31.

Once the metallization steps are completed, the panel is singulated toform the first and second direct metal bonded ceramic substrateassemblies 16 and 21. The first and second direct metal bonded ceramicsubstrate assemblies 16 and 21 in this case are Direct Bonded Aluminum(DBA) substrate assemblies, but in other embodiments the direct metalbonded substrate assemblies 16 and 21 may be Direct Bonded Copper (DBC)substrate assemblies. A stamped metal leadframe is ultrasonically weldedto the DBAs 16 and 21 so that portions 32-39 of metal connects the twoDBA assemblies 16 and 21 together. The diagram of FIG. 8 is asimplification in this regard. At this point in the process, the leadsof the metal leadframe have not yet been separated nor have they beenleadformed, but rather the strips of bridging metal and the metal leadsshown in FIG. 8 are still parts of a single leadframe.

Next, the first direct metal bonded ceramic substrate assembly 16 isbonded (step 104) to the first die 9 so that a direct silver-to-silverbond is formed between the silver 15 on the backside of the die 9 andthe sintered silver pad 28 on the first direct metal bonded ceramicsubstrate assembly. The second direct metal bonded ceramic substrateassembly 21 is bonded to the second die 8 so that a directsilver-to-silver bond is formed between the silver 12 on the backside ofthe die 8 and the sintered silver pad 29 on the second direct metalbonded ceramic substrate assembly 21. To perform this die attach step,the dice 9 and 8 are placed down onto corresponding silver pads 28 and29 of the direct metal bonded ceramic substrate assemblies, and the diceare pressed down under adequate pressure and temperature and held for atime such that the silver layers 15 and 12 on the backsides of the diceare direct silver-to-silver bonded to the sintered silver pads 28 and 29(see FIG. 8) of the direct bonded ceramic substrate assemblies. FIG. 9is a perspective diagram that shows the resulting structure with thedice 9 and 8 attached to the two DBAs 16 and 21, respectively.

Next, the second direct metal bonded ceramic substrate assembly 21 isbonded (step 105) to the first die 9 so that a silver-to-silver bondsare formed between sintered silver pads 13 and 14 on the frontside ofthe die 9 and corresponding sintered silver pads 30 and 31 on the seconddirect metal bonded ceramic substrate assembly 21. Similarly, the firstdirect metal bonded ceramic substrate assembly 16 is bonded to the die 8so that a silver-to-silver bonds are formed between sintered silver pads10 and 11 on the frontside of the second die 8 and correspondingsintered silver pads 26 and 27 on the first direct metal bonded ceramicsubstrate assembly 16. In one example, to perform this attaching step,the structure is folded as illustrated in FIG. 10, and is then pressedtogether under adequate pressure and temperature and held in this wayfor an adequate time as illustrated in FIG. 11 until the directsilver-to-silver bonds are formed. The pressure applied may be a weightof one kilogram applied per die (30 MPa), and the temperature may be250° C., and the time may be two minutes. Due to the folding, each ofthe portions of metal 35, 36 and 37 becomes a bent strip of leadframemetal having on end attached to the first DBA 16 and having a second endattached to the second DBA 21.

The dice are encapsulated in an amount of injection molded encapsulant40 so that a body portion 41 of a packaged power device 42 is formed.Leadforming and leadtrimming are then performed. Alternatively, theleadforming step can be performed before encapsulation, with theleadtrimming step following encapsulation. FIG. 12 is a cross-sectionaldiagram of the resulting packaged power device 42. The metal of theleadframe has been bent (formed) and then trimmed so that five,parallel-extending, single in-line terminals 34, 39, 33, 38 and 32extend downward from the bottom side of the body portion 41. The threebent over strips 35, 36 and 37 of the leadframe metal are disposedentirely within the body portion 41 as illustrated.

FIG. 13 is a circuit diagram of the circuit of the packaged power device42.

FIG. 14 is a perspective diagram of the finished packaged power device42. The finished packaged power device 42 has exposed ceramic of bothDBAs 16 and 21 on outside surfaces of the body portion 41 of the packageso that heat can be better dissipated through these major opposingoutside ceramic surfaces of the body portion. There are no soft solderjoints of any type within packaged power device 42, nor are there anybond wires in packaged power device 42. The metal of the terminals(leadframe metal) are ultrasonically welded to the DBAs without the useof any soft solder. The dice within the package are attached to the DBAsusing direct solderless silver-to-silver bonds. Rather than DBAsubstrates, other types of direct bonded metal (DBM) substrates may beemployed to realize the packaged power device structure of FIG. 14, suchas for example DBC substrates. The metal of the islands as such DBMsubstrates may involve multiple layers of different metals.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

What is claimed is:
 1. A method, comprising: (a) attaching a sinteredsilver pad of a first Direct Metal Bonded (DMB) ceramic substrateassembly to a layer of silver that covers a backside of a semiconductordie such that a silver-to-silver bond is formed between the first DMBceramic substrate and the die, wherein the die comprises the layer ofsilver on the backside of the die, and wherein the die further comprisesa plurality of sintered silver pads on a frontside of the die; and (b)attaching a plurality of sintered silver pads of a second DMB ceramicsubstrate assembly to the plurality of sintered silver pads on thefrontside of the semiconductor die such that a plurality ofsilver-to-silver bonds is formed between the second DMB ceramicsubstrate assembly and the die.
 2. The method of claim 1, furthercomprising: (c) applying an amount of encapsulant to the first andsecond DMB ceramic substrate assemblies thereby forming a body portionof a package; and (d) trimming a first plurality of leads attached tothe first DMB ceramic assembly, and trimming a second plurality of leadsattached to the second direct bonded ceramic assembly, such that thefirst and second pluralities of leads form a single in-line set ofpackage terminals that extends from the body portion of the package. 3.The method of claim 2, wherein a surface of the first DMB ceramicsubstrate assembly is a first surface of the body portion, and wherein asurface of the second DMB ceramic substrate assembly is a second surfaceof the body portion.
 4. The method of claim 1, wherein the forming ofthe silver-to-silver bond in (a) involves pressing the die and the firstDMB ceramic substrate assembly together at an elevated temperature of atleast 250 degrees Celsius, and wherein the forming of thesilver-to-silver bonds in (b) involves pressing the die and the secondDMB ceramic substrate assembly together at an elevated temperature of atleast of at least 250 degrees Celsius.
 5. The method of claim 1, furthercomprising: (c) forming the semiconductor die, wherein the forming of(c) occurs before the attaching of (a) and occurs before the attachingof (b).
 6. The method of claim 5, wherein the forming of thesemiconductor die in (c) involves: (c1) depositing a layer of silveronto an entire backside of a wafer using an evaporative depositionprocess; (c2) after the depositing of (c1) printing silver nanoparticlepaste onto selected portions of a frontside of the wafer; (c3) sinteringthe silver nanoparticle paste on the frontside of the wafer therebyforming the plurality of sintered silver pads; and (c4) dicing the waferthereby forming the semiconductor die.
 7. The method of claim 6, whereinthe selected portions of the frontside of the wafer to which the silvernanoparticle paste is applied in (c2) are aluminum pads that are coveredwith thin layers of aluminum oxide.
 8. The method of claim 1, whereinthe first DMB ceramic substrate assembly is a Direct Bonded Copper (DBC)substrate assembly.
 9. The method of claim 1, wherein the first DMBceramic substrate assembly is a Direct Bonded Aluminum (DBA) substrateassembly.
 10. The method of claim 1, further comprising: (c)encapsulating the first and second DMB ceramic substrate assemblies toform a packaged power device such that a surface of the first DMBceramic substrate forms an outside surface of the packaged power deviceand such that a surface of the second DMB ceramic substrate formsanother outside surface of the packaged power device.
 11. The method ofclaim 10, wherein the packaged power device comprises no soft solder andno bond wires.
 12. The method of claim 1, further comprising: (c)depositing the layer of silver on the backside of the semiconductor dieusing an evaporative deposition process, wherein the depositing of (c)occurs before the attaching of (a) and the attaching of (b).
 13. Themethod of claim 1, further comprising: (c) depositing a layer of silveron the backside of a semiconductor wafer using an evaporative depositionprocess; (d) dicing the semiconductor wafer to form the semiconductordie, wherein the depositing of (c) and the dicing of (d) occur beforethe attaching of (a) and the attaching of (b).
 14. The method of claim1, further comprising: (c) ink-jet printing silver nanoparticle pasteonto selected portions of the frontside of the die; and (d) sinteringthe silver nanoparticle paste on the frontside of the die to form theplurality of sintered silver pads, wherein the ink-jet printing of (c)and the sintering of (d) occur before the attaching of (a) and theattaching of (b).
 15. The method of claim 1, wherein the semiconductordie further comprises a plurality of aluminum pads disposed on thefrontside of the die, and wherein each of the plurality of sinteredsilver pads that is disposed on the frontside of the die is disposed ona corresponding one of the plurality of aluminum pads.
 16. A method,comprising: depositing a layer of silver onto a backside of asemiconductor die using an evaporative deposition process; printingsilver nanoparticle paste onto selected portions of a frontside of thesemiconductor die; sintering the silver nanoparticle paste on thefrontside of the semiconductor die to form a plurality of sinteredsilver pads; forming a silver-to-silver bond between a first DirectMetal Bonded (DMB) ceramic substrate and the semiconductor die byattaching a sintered silver pad of the first DMB ceramic substrate tothe layer of silver on the backside of the semiconductor die; andforming a plurality of silver-to-silver bonds between a second DMBceramic substrate and the semiconductor die by attaching a plurality ofsintered silver pads of the second DMB ceramic substrate to theplurality of sintered silver pads on the frontside of the semiconductordie.
 17. The method of claim 16, further comprising: encapsulating thefirst and second DMB ceramic substrates to form a packaged power devicesuch that a surface of the first DMB ceramic substrate forms an outsidesurface of the packaged power device and such that a surface of thesecond DMB ceramic substrate forms another outside surface of thepackaged power device.
 18. The method of claim 17, wherein the packagedpower device comprises no soft solder and no bond wires.
 19. The methodof claim 17, wherein a bent strip of leadframe metal is disposedentirely within the packaged power device, wherein the bent strip has afirst end that is welded to the first DBM ceramic substrate, and whereinthe bent strip has a second end that is welded to the second DBM ceramicsubstrate.
 20. The method of claim 16, further comprising:ultrasonically welding metal package terminals to the first and secondDMB ceramic substrates.